Offshore loading vessels have for many years employed drum winches to draw in and hold lines, particularly the long mooring lines used to anchor an offshore loading vessel to the sea bottom. As the depths where offshore loading vessels are used have increased, the drum winches used to draw in and store the mooring lines have been pushed toward their limits. This has occurred in two ways. First, the size of the winch permits it to hold between its end flanges only a certain number of wraps of a line having a given line diameter. This means that only a limited length of line can be accommodated before the winch simply becomes incapable of holding any further line that it may draw in. Second, as greater lengths of line are used and stored on the drum, the winch loses mechanical advantage and requires greater driving force in order to be turned. This need for greater driving force means greater wear and tear on motors, power trains, driving pinions and other components involved in turning the drum. It may also lead to breakdown.
Motors and other components can be upgraded. In certain situations, additional drive mechanisms can be added to upgrade existing drive mechanisms. All these solutions can involve one or more disadvantages. First, any changes in an existing drum mooring winch may result in throwing the original design out of balance and causing new problems. Also, it is normally quite expensive to move the entire offshore loading vessel to a shipyard or other such location for such work. Further, if the drum winch alone is to be brought to the shipyard, it must be taken out of service, removed from its mounting, transported, upgraded, transported back, remounted and restored to service. Finally, if the upgrading work is done out on the offshore loading vessel, this must often be done under harsh conditions and with the limited resources available on the offshore loading vessel for moving heavy equipment. Thus, complex and elaborate winch upgrading work poses difficulties. Moreover, the design of drum winches, even if upgraded with adequate power, is such that they will always have a capacity limit based on their size and the amount of line they can hold.
Traction winches are a known solution to the problem of line capacity, because they need only hold a few wraps of line, feeding the line drawn in to a take-up reel. However, traction winches have two drums and a take-up reel, a fundamentally different design. They cannot be directly substituted for a drum winch in most situations. Installing the two drums for a traction winch can lead to significant reworking of the mounting and driving structures, with all the disadvantages mentioned above encountered in replacement or upgrading of a drum winch. It may also require accommodations for the way line loads and various other forces present at the traction winch differ from the forces applied by the replaced winch drum.
It would be desirable to find an effective and less complicated way to upgrade a drum winch to provide increased mooring capability.